Anionic species with ion pair forming ability are commonly used to enhance the retention and efficiency of basic analytes in RPLC separations. However, little is known about the interactions between organic mobile phase modifiers and such ion pairing anions. In this work, we measured the magnitude of the retention increase of basic drugs and peptides upon addition of strong inorganic ion pairing anions (e.g. perchlorate) as a function of the volume fraction of modifier in acidic water-acetonitrile mobile phases on two different stationary phases. We found that the increase in retention upon addition of various salts depended strongly on the eluent strength. In general, larger retention increases upon addition of the anion were observed at higher organic fractions. Regression of retention against the volume fraction of organic modifier indicated that the ion pair forming anions substantially decreased S values while only slightly changing ln k′w values. The decrease in S is the major cause of the retention increase of basic drugs and peptides when such anions are added to the mobile phase.
Bibliographical noteFunding Information:
We thank Professors David V. McCalley and Fred F. Cantwell for many insightful comments. We thank Professor Sarah C. Rutan for her help on statistical analysis of the data. We also acknowledge the financial support from the National Institute of Health (Grant #5R01GM054585-09). We are thankful to Agilent Technologies for donating the Zorbax SB-C18 column, Mac-Mod Analytical Inc. for donating the ACE C18 column, and Systec Inc. for donating the column heater.
- Anionic additives
- Basic drugs
- Ion pair formation
- Retention behavior